DL-based frequency extrapolation can also be used to infer the downlink channel from the uplink one in FDD massive MIMO, which solves the problem resulted from the fact that the channel is not reciprocal in the two different frequency bands. In fact, the technology of multiple bands transmission is a new trend, especially that recent communications systems are designed to work at both sub-6 GHz and mmWave bands, simultaneously [1]}, [2]}, [3]}. In the dual band system, the mmWave spectrum would provide a high speed link and offer gigabit-per-second data rates, but it would also face huge training overhead and high sensitivity to blockages. Since the mmWave and sub-6 GHz links experience the same scattering environment, it is possible to build a deterministic relationship between their channels and extrapolate the CSI of mmWave band from that of the sub-6 GHz band.
